1. Field of the Invention
The present invention relates to a pixel circuit having an electro-optical element in which a luminance is controlled by a signal line, a display device, such as an organic EL (electroluminescence) display, an LCD (liquid crystal display) device, or other active matrix display device, in which a plurality of the pixel circuit is arranged in matrix, and a method for controlling the pixel circuit.
2. Description of the Related Art
In the active matrix display device, a liquid crystal cell, an organic EL element, or other electro-optical elements is used as a display element in a pixel.
Among them, the organic EL element has a structure in which a layer made of an organic material, namely an organic layer, is sandwiched between electrodes.
When the organic EL element is applied with a voltage, electrons are injected from a cathode into the organic layer and holes are injected from an anode into the same, as a result, electrons and holes are re-coupled to emit light. The organic EL element has the following merits.
(1) A luminance of several 100 to several 10,000 cd/m2 is obtained by driving at a low voltage of 10 V or less, so a low power consumption is possible.
(2) A contrast of an image is high due to a self-luminescence element and the response speed is fast, so a viewability is good and the element is suitable for a moving image display.
(3) The element is formed by an all solid state element having a simple structure, so a high reliability and thinness of the element can be realized.
An organic EL display device (hereinafter, referred to as organic EL display) in which the organic EL element having the above merits is used as the display element of a pixel, has been gathered attention as a next generation flat panel display.
As a drive system of the organic EL display, there are a simple matrix system and an active matrix system. In the above systems, the active matrix system has the following merits.
(1) The active matrix system by which an emitting light of the organic EL element each in a pixel is able to be retained during a single frame period, is suitable for a high definition and a high intensity of an organic EL display.
(2) A peripheral circuit used a thin film transistor is able to be formed on a substrate (panel), so an interface of the panel to an external can be simplified and a high function panel is possible.
In the active matrix organic EL display, a thin film transistor (hereinafter, referred to polysilicon TFT) having an active layer formed by polycrystalline silicon is generally used as a transistor of an active element.
This is because the polysilicon TFT has a high drivability and a pixel size can be designed to be small, which are advantageous to a high definition.
On the other hand, it is known that polysilicon TFT has a large variation of properties while having the merits described above.
Therefore, when the polysilicon TFT is used, suppressing the variation of property and compensating the variation of property of the TFTs by a circuit are considerable disadvantages for the active matrix organic EL display used with the polysilicon TFT. This is because of the following reason.
Namely, while a liquid crystal display used with a liquid crystal cell as the display element in the pixel adopts a configuration for controlling a luminance data each in the pixel by a voltage value, the organic EL display adopts a configuration for controlling the same by a current value.
A schema of the active matrix organic EL display will be described.
FIG. 1 is a schematic view illustrating a configuration of a general active matrix organic EL display, and FIG. 2 is a circuit diagram illustrating a configuration example of a pixel circuit of the active matrix organic EL display (for example, referred to U.S. Pat. No. 5,684,365 and Japanese Unexamined Patent Application (Kokai) No. 8-234683).
In an active matrix organic EL display 1, an m×n number of pixel circuit 10 is arranged in matrix, and, with respect to a matrix arrangement of the pixel circuit 10, an n number of row's worth of signal lines SGL1 to SGLn driven by a data driver (DDRV) 2 is interconnected in each pixel row and an m number of columns of scan lines SCNL1 to SCNLm driven by a scan driver (SDRV) 3 is interconnected in each pixel column.
The pixel circuit 10 has a p-channel TFT 11, an n-channel TFT 12, a capacitor C11, and a light emitting element 13 formed by an organic EL element (OLED) as shown in FIG. 2.
In the TFT 11 of the pixel circuit 10, a source is connected to a power source potential line VCCL and a gate is connected to a drain of the TFT 12. In the organic EL light emitting element 13, an anode is connected to a drain of the TFT 11 and a cathode is connected to a reference potential (for example, a grand potential) GND.
In the TFT 12 of the pixel circuit 10, a source is connected to one of the signal line SGL1 to SGLn of a corresponding row, and a gate is connected to one of the scan lines SCNL1 to SCNLm of a corresponding column.
In the capacitor C11, an end is connected to the power source potential line VCCL and another end is connected to the drain of the TFT 12.
Note that, the organic EL element usually has a rectification property in many cases, so sometimes is called as an organic light emitting diode (OLED). Although FIG. 2 or other drawings illustrate the light emitting element by using a symbol of a diode, the rectification property may be not demanded to the OLED in the following explanation.
In the pixel circuit 10 having the above configuration, a pixel row including a pixel in which the luminance data is written is selected by the scan driver 3 via the scan line SCNL, as a result, the TFT 12 of the pixel in the row is turned on.
At this time, the luminance data is supplied as a voltage from the data driver 2 via the signal line SGL, and is written via the TFT 12 into the capacitor C11 for holding a data voltage.
The luminance data written into the capacitor C11 is held during a single field period. The held data voltage is applied to the gate of the TFT 11.
Therefore, the TFT 11 drives the organic EL light emitting element 13 by current based on the held data. At this time, a gray-scale display of the organic EL light emitting element 13 is carried out by modulating a gate-source voltage Vdata (<0) of the TFT 11 held by the capacitor C11. a
Generally, a luminance Loled of the organic EL element is proportional to a current Ioled flowing therein. Therefore, a relationship between the luminance Loled and the current Ioled of the organic EL light emitting element 13 is expressed by the following formula (1).Loled∝Ioled=k(Vdata−Vth)2  (1)
In the above formula (1), “k=½·μ·Cox·W/L” stands. Here, “μ” indicates a mobility of a carrier of the TFT 11, “Cox” indicates a gate capacitance per unit area of the TFT 11, “W” indicates a gate width of the TFT 11, and “L” indicates a gate length of the TFT 11.
Therefore, it is understood that each variation of the mobility μ and a threshold voltage Vth (<0) of the TFT 11 directly influences a variation of a luminance of the organic EL light emitting element 13.
In this case, when the same voltage Vdata for example is written into different pixels, the threshold Vth of the TFT 11 varies each in the pixel, as a result, the current Ioled flowing into the light emitting element (OLED) 13 largely varies each in the pixel and then the current is completely off the desirable value, so a high image quality may not be expected as a display.
To overcome the above disadvantages, various pixel circuits have been proposed and a typical example is illustrated in FIG. 3 (another example, refer to U.S. Pat. No. 6,229,506 or Japanese Unexamined Patent Application (Kokai) No. 2002-514320 of FIG. 3).
A pixel circuit 20 shown in FIG. 3 has a p-channel TFTs 21, to 24, capacitors C21 and C22, and an organic EL light emitting element 25 as the light emitting element. In FIG. 3, “SGL” indicates a signal line, “SCNL” indicates a scan line, “AZL” indicates an auto-zero line, and “DRVL” indicates a drive line.
An operation of the pixel circuit 20 will be explained with reference to timing charts illustrated in FIGS. 4A to 4E.
As shown in FIGS. 4A and 4B, the drive line DRVL and the auto-zero line AZL are set at a low level to make the TFT 22 and the TFT 23 conductive states. At this time, the TFT 21 with a diode connected is connected to the light emitting element (OLED) 25, so a current flows into the TFT 21.
As shown in FIG. 4A, the drive line DRVL is set at a high level to make the TFT 22 a non-conductive state. At this time, the scan line SCNL is set at the low level shown in FIG. 4C to make the TFT 24 the conductive state, and the signal line SGL is applied with a reference potential Vref shown in FIG. 4D. The current flowing into the TFT 21 is cut off, so a gate potential Vg of the TFT 21 rises, as shown in FIG. 4E. At the point where the gate potential Vg rises up to “VDD−|Vth|”, the TFT 21 is turned off and the potential is stabilized. The above operation is also referred to as an “auto-zero operation” in the following.
As shown in FIGS. 4B and 4D, the auto-zero line AZL is set at the high level to make the TFT 23 the non-conductive state, as a result, a potential of the signal line SGL falls at “ΔVdata” from “Vref”. A change of the signal line potential lowers the gate potential of the TFT 21 at “ΔVg” via the capacitor C21 as shown in FIG. 4E.
As shown in FIGS. 4A and 4C, the scan line SCNL is set at the high level to make the TFT 24 the non-conductive state and the drive line DRVL is set at the low level to make the TFT 22 the conductive state. As a result, the current flows into the TFT 21 and the light emitting element (OLED) 25, and then the light emitting element 25 starts to emit light.
If a parasitic capacitance can be disregard, “ΔVg” and the gate potential Vg of the TFT 21 are expressed by the following formulas.ΔVg=ΔVdata×C1/(C1+C2)  (2)Vg=Vcc−|Vth|−ΔVdata×C1/(C1+C2)  (3)
Here, “C1” indicates a capacitance of the capacitor C21, and C2” indicates a capacitance of the capacitor C22.
On the other hand, when a current flowing into the light emitting element (OLED) 25 as emitting light is defined as “Ioled”, the current is controlled by the TFT 21 connected to the light emitting element 25 in serial. If it is assumed that the TFT 21 operates in a saturate region, the following relationship is obtained by applying a well-known formula of the MOS transistor and the above formula (3).
                                                        Ioled              =                              μCox                ⁢                                                                  ⁢                                                      W                    /                    L                                    /                  2                                ⁢                                                                  ⁢                                                      (                                          Vcc                      -                      Vg                      -                                                                      Vth                                                                                      )                                    2                                                                                                        =                              μCox                ⁢                                                                  ⁢                                                      W                    /                    L                                    /                  2                                ⁢                                                                  ⁢                                                      (                                          Δ                      ⁢                                                                                          ⁢                      Vdata                      ×                      C                      ⁢                                                                                          ⁢                                              1                        /                                                  (                                                                                    C                              ⁢                                                                                                                          ⁢                              1                                                        +                                                          C                              ⁢                                                                                                                          ⁢                              2                                                                                )                                                                                      )                                    2                                                                                        (        4        )            
Here, “μ” indicates the mobility of a carrier, “Cox” indicates a gate capacitance per a unit area, “W” indicates a gate width, and “L” indicates a gate length.
According to the formula (4), “Ioled” does not depend on the threshold Vth of the TFT 21 and is controlled by “ΔVdata” given from an external. In other words, by using the pixel circuit 20 shown in FIG. 3, a display device having relatively high uniformity both current and in luminance can be realized without being affected by the threshold Vth varying each in the pixels.